In recent years, a visible light communication system (visible light ID system) is put in practice use, which is configured to modulate visible light (illumination light) emitted from a luminaire (transmitter), to transmit unique ID information and the like to the luminaire, and demodulate the ID information with a receiver that receives the visible light.
Here, a visible light communication apparatus described in JP 2012-69505 A (referred to as “document 1”) is illustrated as a conventional example of the transmitter that constitutes the visible light communication system. As showed in FIG. 7, this visible light communication apparatus 10 includes a constant current source 1, a smoothing capacitor 2, an LED source 3, a load change element 4, a signal generating circuit 6, and a switch element 5.
The smoothing capacitor 2 is connected between output ends of the constant current source 1 and is configured to smooth an output of the constant current source 1. The LED source 3 includes a plurality of light emitting diodes connected in series between the output ends of the constant current source 1, and the output of the constant current source 1, which is smoothed by the smoothing capacitor 2, is supplied. The load change element 4 is for partially changing load characteristic of the LED source 3 by being added to the LED source 3. For example, the load change element 4 includes a resistor connected in parallel to a part of the plurality of light emitting diodes. The signal generating circuit 6 is configured to generate an optical-communication signal of a binary. The switch element 5 includes a switching element (for example, a MOSFET) connected in series with the resistor that constitutes the load change element 4, for example. It is switched whether the switch element 5 is configured to switch whether or not the load change element 4 is added to the LED source 3 by switching ON/OFF by the optical-communications signal of a binary.
That is, the size of a load current that flows through the light emitting diodes in a time period when the load change element 4 is not added to the LED source 3 is different from that of a time period when the load change element 4 is added to the LED source 3, and therefore, the optical-communication signal can be superimposed on the light quantity of the visible light communication apparatus 10 and be transmitted.
The optical-communication signal transmitted from the visible light communication apparatus 10 is received by a receiver 20 that includes a photo IC. The receiver 20 adopts the system of receiving the optical-communication signal by determining the difference between an optical output on which the optical-communication signal is not superimposed and an optical output on which the optical-communication signal is superimposed. By adopting the above system, it is possible to detect modulated light even if the modulated light is small.
Incidentally, a specification (protocol) of the visible light communication is specified to Japan Electronics and Information Technology Industries Association standard JEITA CP-1222 (visible light ID system), for example. In this standard, send data is converted to a pulse position each unit that includes two bits. Such a conversion (coding) system is called 4 PPM (pulse position modulation). In the 4 PPM coding, the signal generating circuit 6 turns on the switch element 5 to increase the optical output (light on) when a slot is set to “0”. The signal generating circuit 6 turns off the switch element 5 to decrease the optical output (or light off) when a slot is set to “1” (refer to FIG. 2).
Here, in the above-mentioned standard, a pattern of three slots that are assigned with “1” continuously and nine slots that are assigned with “0” continuously (“111000000000”) is used as a preamble for specifying a starting position of a frame (refer to FIG. 8). The pattern does not occur even if the 4 PPM coding is adopted to the data. When a fourth slot of the data transmitted immediately before the preamble is assigned with “1”, four slots assigned with “1” are continuously constituted of the fourth slot and the three continuous slots assigned with “1” starting from a head of the preamble. Because the data speed in the above-mentioned standard is 4.8 kilobits per second, the time period for one slot is 104.2 μs, and the time period for the four slots is 416.7 μs that is nearly equal to 104.2×4.
Then, because the discharging amount of the electric charge charged in the smoothing capacitor 2 increases when the continuous off period of the switch element 5 is extended to the time period for the four slots, a waveform of the load current may be disturbed largely when a change to “0” from “1” of the last is performed (refer to a time period T in FIG. 9). Thus, when the waveform of the load current is disturbed largely, there is a possibility that the optical-communication signal is not modulated correctly and then a communication error occurs. It is not desirable that the electrostatic capacitance of the smoothing capacitor 2 is enlarged in order to solve such a problem because the dimension of the smoothing capacitor 2 enlarges and the component cost of the smoothing capacitor 2 rises.